The field of chemistry and biology continues to advance at a rapid pace. New chemical and biological agents are developed daily in laboratory settings. However, conventional processing equipment suffers from a number of disadvantages. It has long been recognized in the chemical industry that “scale up” from laboratory bench-scale to commercial production scale is difficult. Results achieved in the laboratory are often difficult to duplicate at production rates in production facilities. Methods of controlling and optimizing processes for producing such chemical and biological compounds are becoming better understood. The control of parameters such as temperature, pressure, mixing conditions, relative volumes of reactants, and uses of catalysts are also becoming better understood. Traditionally, newly discovered chemical and biological compounds and processes involving either the production of such compounds, or processes involving the use of such compounds, have initially been carried out in “bench-scale” environments. Promising chemicals, biological agents, or processes are ultimately produced in mass quantity by application to industrial-scale processes. However, problems are often encountered in scaling up the process from the laboratory to industrial-scale production.
Conventional chemical processing equipment typically holds a relatively large volume of materials and consequently has a relatively large volume to surface area ratio. As a result, different portions of the reactant materials contained within such equipment are exposed to different histories of conditions. In the case of a conventional tank reactor, for example, even when temperature conditions at the walls of the reactor are well controlled, the portions of the reactants that are not in close proximity to the walls of the reactor may experience different temperature histories, especially if a significant temperature gradient exists, which might occur if the chemical reaction is strongly exothermic. Rapid stirring of the reactants may reduce this temperature history difference, but will not eliminate it. As a result of the nonhomogeneous temperature history, different portions of the reactants may chemically react differently. Undesired reactions may occur in portions of the reactants that are exposed to histories of higher than desired temperatures. This may result in the production of undesired waste products, which may be hazardous and which must be properly disposed of. In extreme situations reaction rates may accelerate to uncontrollable levels, which may cause safety hazards, such as potential explosions. If, however, the volume to surface area ratio of the processing apparatus is substantially reduced, the degree of precision of control of homogeneity of temperature history of the reactants can be substantially improved.
Other common problems associated with moving from bench-scale production to industrial-scale production involve changes in process conditions between the bench-scale environment and the industrial environment. For instance, the temperature of the reactants in a beaker or flask in a laboratory is easier to keep constant than the temperature in a production tank having a capacity of hundreds of gallons, as is often the case in a chemical processing plant. In addition, high pressures and temperatures are easier to maintain in small laboratory sized vessels than in much larger vessels used for production scale operation. In many instances, it is cost prohibitive or not feasible to scale up a reaction vessel from a bench-scale environment to industrial-scale processes. Variations in other process conditions within a large tank are also more difficult to control, and frequently affect the quality and yield of the desired product.
Another aspect of laboratory development of processes to produce chemical or biological compounds is that often potentially dangerous chemicals are used to create the desired product. Fires and explosions in research laboratories and concomitant injury to personnel and property are well known risks, especially in the chemical research industry. The risks are not limited only to research, since industrial chemical or biological production facilities also may experience fires and explosions related to chemical production using dangerous chemicals. Often, due to the quantities of chemicals used in industrial-scale processes, such accidents are significantly more devastating in an industrial setting than similar accidents in a research setting.
The materials of construction of conventional chemical processing apparatus, such as steel and specialty iron alloys, furthermore may be subject to corrosion and wear, may have undesirable effects on catalytic activity, or may “poison” a catalyst.
It has been recognized that a high degree of flow turbulence enhances the ability to rapidly mix two or more reactants together. Rapid mixing is important for fast-acting chemical reactions. A high degree of turbulence is also known to enhance heat transfer. Thus, a structure having both a low volume to surface area ratio and a high degree of flow turbulence is particularly advantageous for precise control of chemical processing.
Recently, increased attention has been directed to the use of micro-reactors for both development and production of chemical and biological processes. These types of reactors offer several advantages. As stated above, the control of chemical processes within very small reactors is typically easier than the control of a similar process in a large-scale production tank. Once a reaction process has been developed and optimized in a micro-reactor, it can be scaled up to industrial production level by replicating the micro-reactors in sufficient quantity to achieve the required production output of the process. If such reactors can be fabricated in quantity, and for a modest cost, industrial quantities of a desired product can be manufactured with a capital expenditure equal to or even less than that of a traditional chemical production facility. An additional benefit is that because the volume of material in each individual reactor is small, the effect of an explosion or fire is minimized, and with proper design, an accident in one reactor can be prevented from propagating to other reactors.
The use of micro-reactors has also resulted in an increase in safety in laboratory settings. In the research setting, the use of micro-reactors generally results in less exposure to hazardous substances and conditions by research personnel than when using traditional “batch chemistry” equipment, which equipment typically requires the researcher to physically handle chemicals in a variety of glass containers, often in the presence of a heat source. An accident in such an environment is likely to increase the risk of exposure to hazardous chemicals, and cause damage to the laboratory. However, when using a micro-reactor, the micro-reactor is typically a self-contained unit that minimizes the researcher's potential exposure to chemical substances. When using a micro-reactor, the researcher is not required to physically manipulate containers of chemical materials to carry out a desired reaction. As such the micro-reactor can be located in an area that will protect the researcher from an accident that could result in a fire or explosion.
Another area in which micro-reactors offer an advantage over conventional chemical process development and production is in the mixing of reactants. A mixing channel of the proper scale encourages a laminar flow of the reactants within the channel and is readily achievable in a micro-reactor. Laminar flow can enhance mixing by diffusion, which can eliminate the need to expend energy to physically stir or agitate the reactants.
Micro reactors are particularly applicable to the pharmaceutical industry, which engages in chemical research on many new chemical compounds every year, in the effort to find drugs or chemical compounds with desirable and commercially valuable properties. Enhancing the safety and efficiency of such research is valuable. When coupled with the potential that micro-reactors offer for eliminating the problems of moving from bench-scale production to industrial production, it is apparent that a micro-reactor suitable for use in carrying out a variety of chemical processes, and having an efficient and low cost design is desirable.
Several different designs for micro-reactors have been developed. Some of these designs are disclosed in U.S. Pat. Nos. 3,701,619; 5,534,328; 5,580,523; 5,690,763; 5,961,932; and U.S. patent application Ser. Nos. 2002/0106311 published Aug. 8, 2002, 2002/0048644 published Apr. 25, 2002; and 2003/0091496 published May 15, 2003. All of these patents and patent applications are incorporated herein by reference for teachings concerning reactors, materials used to manufacture the reactors, techniques used to manufacture the reactors, and catalysts used in association with the reactors.
One example of a micro-reactor is disclosed in U.S. Pat. Nos. 5,534,328 and 5,690,763, both of which are incorporated herein by reference. These two patents describe reactor structures for chemical manufacturing and production, fabricated from a plurality of interconnected layers. Generally, each layer has at least one channel or groove formed in it and most include orifices that serve to connect one layer in fluid communication with another. These layers are preferably made from silicon wafers, because silicon is relatively inert to the chemicals that may be processed in the reactor, and because the techniques required to mass produce silicon wafers that have had the required channels and other features etched into their surfaces are well known. A disadvantage of the micro-reactors described in the two patents stems from the rather expensive and complicated process required for manufacturing the devices. While silicon wafer technology has advanced to the state that wafers having desired surface features can readily be mass produced, the equipment required is capital intensive, and unless unit production is extremely high, the substantial costs are difficult to offset. While the two patents suggests that other materials can be used to fabricate the layers, such as metal, glass, or plastic, the surface features required (grooves, channels, etc.) must still be formed in the selected material. The specific surface features taught by the two patents require significant manufacturing steps to fabricate. For instance, while forming an opening through a material is relatively easy, forming a groove or channel that penetrates only part way through the material comprising a layer is more difficult, as the manufacturing process must not only control the size of the surface feature, but the depth as well. When forming an opening that completely penetrates through a material comprising a layer, depth control does not need to be so precisely controlled. The two patents teach that both openings which completely penetrate the layers, and surface features (grooves/channels) that do not completely penetrate the individual layers are required. Hence, multiple processing steps must be employed in the fabrication of each layer, regardless of the material selected.
U.S. Pat. No. 5,580,523, which is incorporated herein by reference, describes a modular micro-reactor that includes a series of modules connected in fluid communication, each module having a particular function (fluid flow handling and control, mixing, chemical processing, chemical separation, etc.). The patent teaches that the plurality of modules are mounted laterally on a support structure, and not stacked. In a preferred embodiment of the invention, silicon wafer technology is again used to etch channels and/or other features into the surface of a silicon wafer. Other disclosed fabrication techniques include injection molding, casting, and micro-machining of metals and semiconductor substrates. Again, the processing required to fabricate the individual modules goes beyond merely forming a plurality of openings into each component. Furthermore, the lateral layout of the reactor described in the patent requires a larger footprint (Basis Area) than a stacked plate reactor. The reactor requires more modules, thus a larger footprint of the entire reactor is required. In contrast, when additional plates are added to a stacked plate reactor, the footprint of the reactor does not change, which can be a distinct advantage, as in many work environments, the area an apparatus occupies on a workbench or floor is more valuable than the vertical height of the apparatus. As such, the disclosed reactor does not minimize the footprints and still provides flexibility to add components to customize the reactor for a particular process or application.
U.S. Pat. No. 5,961,932, which is incorporated herein by reference, discloses a reactor that is formed from a plurality of ceramic layers, which are connected in fluid communication, and wherein at least one layer includes a permeable partition. In the preferred embodiment, the patent describes that channels and passageways are formed in each layer. The particular process involves fabricating the layers from “green” or uncured ceramic, which once shaped as desired, must be sintered. The sintering process changes the size of the ceramic layer so that the sizes of the features formed into the ceramic layer in the initial stages of production are different than in the finished product. One problem with this reactor design is that the dimensions of the individual components cannot be rigidly controlled during fabrication since the components shrink. Such shrinkage can negatively effect the dimensions of the finished reactor. As such, precise dimensional control of fluid pathways in the reactor are difficult to maintain to achieve the desired flow rates through the reactor.
In U.S. patent application Ser. No. 2002/0106311 published Aug. 8, 2002 entitled “Enhancing Fluid Flow in a Stacked Plate Microreactor,” which is incorporated herein by reference, a stacked plate chemical reactor in which simple plates are stacked together to form the reactor is disclosed. The stacked plates include openings that define fluid pathways and processing volumes within the stacked plates. In a preferred embodiment, an n-fold internal array is achieved by providing a first group of simple plates defining a reaction unit that includes bypass fluid channels and reaction fluid channels for each reactant, such that a portion of each reactant is directed to subsequent groups of simple plates defining additional reaction units. A chemical reactor with variable output is obtained by reversibly joining reactor stacks comprising irreversibly joined reaction units, these reaction units consisting of a plurality of simple plates. Other embodiments disclosed in the patent application employ at least one of an array of parallel fluid channels having different widths, bifurcated fluid distribution channels to achieve a substantially even flow equipartition for fluids with varying viscosities flowing within the fluid channels of each reaction unit.
In several of the prior art reactors identified above, relatively complicated manufacturing techniques are required. The manufacture of layers of silicon material requires a large capital investment. Sintering of a ceramic material requires the precise control of the shrinkage process, or individual components of a desired size cannot be achieved. In all cases, these reactors require complicated structures (for example, fluid channels and reaction channels) to be etched or otherwise fabricated in each layer. Additionally, orifices or passages also need to be formed in each layer, so that fluids can move between adjacent layers of the reactor. Thus, a series of different manufacturing steps typically must be performed for each layer. As such, it is desirable to provide a reactor design offering the advantages described above, which is relatively simple to manufacture, so as to minimize capital investment in scaling up production from the laboratory to the industrial production levels.
While a single micro-reactor can produce only a limited volume of product, additional micro-reactors can be added in parallel to increase production capacity. When additional modular micro-reactor units are added, additional systems for reactant supply, heat transfer media supply, and product collection are typically required, which not only increases the complexity of the system, but also requires more space for duplicative fluid systems. Furthermore, even minor differences in feed rates for some of the duplicate reactor modules can negatively effect product quality. Finally, more sophisticated control and monitoring are required to manage additional reaction modules and feed systems. It would therefore be desirable to provide a micro-reactor capable of n-fold parallelization without requiring that additional fluid and control systems be provided.
In an array of identical fluid channels having a single common reactant distribution channel and a single common product collection channel, with the reactant inlet and the product outlet located at opposite ends, where the common reactant distribution and the common product collection channel have the same cross sectional area, if the viscosity of the product relative to the reactants is substantially the same, then the pressure drop through the array can be considered the same, and the resulting flow distribution is fairly even, with only slightly lower flow rates in the central fluid channels. However, the flow distribution through such an array is not even if the viscosity of the product is significantly different than the viscosities of the reactants. When such an array is employed to process a reaction whose product has a significantly different viscosity compared to the viscosity of the mixture of the unreacted reactants, broad residence time distributions result in the array due to the fact that the pressure drop in the common reactant distribution channel no longer balances with the pressure drop in the common product collection channel. The flow rates within each individual fluid channel in the array are no longer identical. If the viscosity of the product is significantly greater than the viscosity of the mixed but unreacted reactants, then the flow rates in the individual fluid channels in the array tend to increase across the array for channels closest to the common product outlet. Thus the highest flow rate is experienced in the fluid channel in the array that is closest to the common product outlet, while the lowest flow rate is experienced in the fluid channel in the array that is located furthest from the common product outlet. This phenomenon is different if the viscosity of the product is less than the viscosity of the mixed but unreacted reactants. Thus for lower viscosity products, the highest flow rate is experienced in the fluid channel in the array that is closest to the common reactant inlet, while the lowest flow rate is experienced in the fluid channel in the array that is located furthest from the common reactant inlet. The greater the relative change in viscosity, the greater the variation in flow rates across the array. This imbalance leads to different residence times being associated with different fluid channels, resulting in an undesirable residence time distribution within the whole reaction unit. In certain cases, the additional residence time can lead to undesired cross reactions, and even clogging of the “slowest” fluid channels. As such, it is desirable to provide a micro-reactor including a plurality of fluid channels that is capable of processing reactant mixtures undergoing a significant viscosity change without the above-described residence time distributions and related problems.
For the specific residence time distributions discussed above, relative to reactant mixtures produced in fluid channels in which a plurality of different reactants are mixed, only one type of undesirable residence time distribution is of concern. Residence time distribution problems of this type can also arise in fluid channels used to direct reactants before mixing, as well as products for collection. It is desirable to provide a micro-reactor that includes a plurality of fluid channels adapted to provide substantially equal residence time distributions for fluid flow within the micro-reactor.
Computer modeling of reactors has increased in popularity due to increased computer processing power and increased sophistication in modeling software. As such, reactors are commonly modeled to have increased complexity (e.g., various passageway configurations for increased reactor residence time; passageway configurations to maintained desired flow patterns, temperature profiles, pressure profiles, etc.). These complex reactor designs are difficult, if not impossible, to manufacture and/or are cost prohibitive to manufacture by use of prior art reactor design techniques. Many chemical manufacturing processes also require exposure to catalytic materials to complete the chemical process. Precious metals such as gold, platinum, palladium, iridium, rhodium, silver and the like are used as catalysts in various chemical reactions. In the past, separate reactors had to be produced that contained each different catalyst material. The use of a plurality of reactors resulted in an increase in cost and complexity of a chemical reactor system.
As a result, there is a need for a micro-reactor that can be economically manufactured, can incorporate unique and sophisticated flow patters through the reactor, can maintain a desired relatively narrow temperature range for a process, has a relatively modest footprint, can provide desired diffusion mixing, can process reaction mixtures that form a product with different viscosities, can provide desired residence time distributions for fluid flow within the micro-reactor, and can include different types of catalytic materials.